Yeon Gaesomun

Yeon Gaesomun (603-666) was Prime Minister of Goguryeo from 642 to 665, preceding Yeon Namsaeng. He seized power after overthrowing and killing King Yeongynu of Goguryeo in a military coup in 642, becoming a  military dictator and fighting off an invasion by Yeongnyu's Tang Chinese allies.

Biography
Yeon Gaesomun was born in 603 to an influential Goguryeo family; he was the son of Prime Minister Yeon Taejo. While serving as Western Governor, he built several military garrisons along the Liaodong border to protect Korea from Tang invasions. In 642, he overthrew and murdered King Yeongnyu of Goguryeo in a military coup after Yeongnyu attempted to have the powerful general assassinated, and he became "Grand Prime Minister" and a military dictator. Yeongnyu was a vassal of Tang China, while Yeon was anti-Chinese, leading to Emperor Taizong of Tang retaliating by launching an invasion of Goguryeo in 645 AD. Yeon led a 150,000-strong army to face the invading 200,000-strong army at the Battle of Mt. Jupil, but his army was destroyed, and he retreated with just 10,000 men remaining in his ranks. He retreated to the capital of Pyongyang and defended it from Tang marines while his rival Yang Manchun defended Ansi Fortress from an eighty-day siege, and he also defeated further Tang invasions in 647 and 648. In 662 AD, Yeon Gaesomun slew the Chinese general Pang Xiaotai and his 13 sons at Sasu River, and he continued to defend Goguryeo until his death in 666 AD. His death caused a succession struggle which weakened Goguryeo until it fell to the Tang in 668 AD.